1. Field of the Invention
This invention relates in general to power amplifiers, and more particularly, to a method and apparatus for compensating for loss of RF output power from the isolation port of a hybrid coupler.
2. Description of Related Art
To meet the increasing RF output power requirements of a cellular base station or other communication systems, it has become a necessity to utilize a parallel amplifier configuration for a final power amplifier stage of a transmitter unit in the communication system. The utilization of a parallel amplifier configuration causes a problem for the transmitter unit in the form of heat dissipation and RF power loss at the output of the RF power coupler, e.g. a power divider, or a power combiner. A block diagram of a parallel amplifier configuration in a typical power amplifier system can be seen in FIG. 1.
In FIG. 1, an input signal from previous stages of a transmitter unit in a communication system, such as a cellular base station, is injected into a first coupler, namely an input signal coupler. A portion of the input signals is coupled and sent to a power control circuitry which monitors and adjusts the input signal for an appropriate signal level. From the first coupler, the signal enters a first hybrid coupler, used as a power divider, whereby the signal is divided. The divided signals vary in amplitude and phase depending on a mismatch presented at the termination of an isolation port of the first hybrid coupler and on the impedance variation presented by the power amplifiers in parallel. The amplitude of the signal that is not realized at either of the output ports of the power divider is realized at the termination of the isolation port of the first hybrid coupler and is dissipated as heat into the resistor present from the first hybrid coupler to the ground, thereby causing power loss at the output ports of the RF power divider. The divided signals that are realized at the output ports of the power divider are amplified by the respective power amplifiers and are presented to a second hybrid coupler, used as a power combiner. The signals are combined, and the resultant signal varies in amplitude and phase depending on the mismatch presented at the termination of the isolation port of the second hybrid coupler and on the impedance variation presented by the power amplifiers. The amplitude of the signal that is not realized at the output port of the power combiner is realized at the termination of the isolation port of the second hybrid coupler and is dissipated as heat into the resistor present from the second hybrid coupler to the ground, thereby causing power loss at the output port of the RF power combiner.
To solve the above problem, U.S. Pat. No. 4,656,434 proposed an RF power amplifier with load mismatch compensation. In the '434 patent, the RF power amplifier delivers power to a load including: a first quadrature hybrid coupler driving two class C amplifiers operating with high input compression; a second quadrature hybrid coupler coupled to the outputs of the amplifiers; an attenuator; and a phase shifter. An impedance mismatch by the load, causing phase-pulling of the amplifiers which reduces output to the load, is compensated for by feeding an error signal. The error signal indicates the magnitude and phase of the phase-pulling of the two amplifiers, from the second quadrature hybrid coupler through the attenuator and the phase shifter back to the first quadrature hybrid coupler. The attenuated and phase shifted error signal adds to or is subtracted from the input signals to the two amplifiers. Therefore, one of the two amplifiers is driven with more signal and the other amplifier is driven with less signal to phase-push the two amplifiers to compensate for the phase-pulling to thereby increase the output to the load.
The problem of the power amplifier system in the '434 patent is that the power control is accomplished by a feedback attenuator. The feedback attenuator basically takes the place of the termination at the isolation port of the second quadrature hybrid coupler and still dissipates the same approximate amount of heat. In addition, the feedback component in the '434 patent is an error signal. Thus, the RF power loss at the final power amplifier stage of the transmitter unit and the heat problem which would cause low reliability of the power amplifiers still exist. Further, the '434 patent does not resolve the power loss and heat dissipation problem of the first quadrature hybrid coupler. Furthermore, in the '434 patent, the fed back error signal drives one of the amplifiers harder than the other. Therefore, one of the harder amplifiers is forced to run at a higher operating junction temperature thereby having a lower MTTF (Mean Time To Failure).
Accordingly, it can be seen that there is a need for a power amplifier system that compensates for the loss of an RF output power. There is also a need for a power amplifier system that reduces heat dissipation at terminations, resistors, or other components of the system.